Biological pretreatments of biomass for improving biogas production: an overview from lab scale to full-scale

Recent shifts in European countries biogas policies tend to limit the use of energy crops and encourage the use of manure, lignocellulosic feedstocks and bio-waste. The need to use feedstocks that are more difficult to handle (displaying either too low or too high biodegradation rates) is calling for the development of adapted pretreatments. Among them, biological pretreatments are very promising due to their reasonable cost, environmental friendliness and possible application to a wide spectrum of feedstocks. They can be divided into three categories: enzymatic, anaerobic and aerobic ones. This review aims at providing some guidelines on which type of biological pretreatment to apply for a given feedstock. To deliver such recommendations we considered the full range of technological readiness level. We gathered an analysis of the recent literature data obtained at lab or pilot scale focusing on methane yield enhancements and the description of some full-scale commercialized technologies. For lignocellulosic feedstocks, both enzymatic pretreatments using lignin-modifying enzymes or carbohydrases and aerobic pretreatments using consortia or simple aeration appear as promising. For bio-waste, anaerobic pretreatment via two-stage digestion seems to be an efficient biological pretreatment. For landfill, enzymatic treatment may be an interesting solution. Finally, for sludge digestion, both aerobic and anaerobic pretreatments favouring autohydrolysis may be recommended. Full-scale applications already exist but their use remains scarce. Indeed, each biological pretreatment features technological issues. Enzymes have high production costs and limited activity in time. Anaerobic pretreatments, notably two-stage digestion, are more expensive and complex to handle than a single stage. Finally, aerobic pretreatments need fine tuning and control due to respiration mass loss. Research and development conducted toward these specific issues may allow these pretreatments to become more cost-effective as well as practical and thus facilitate their development at fullscale.